There is considerable interest in the subunit composition of myosin because of the roles of varied expression in determining contractile properties of myocardium and other muscles. Myosin isoforms can be identified in several ways (reviewed in 54) including separation of the native molecules on non-denaturing gels, identification of distinct species of myosin heavy and light chains by SDS-PAGE and Western blots, and immunoreactivity of fixed thin sections of muscle. Each myosin molecule is comprised of 2 heavy chains (MHC) of about 200 kDa and 4 light chains (MLC) with molecular weights between 16 and 25 kDa (18,69). Previous studies have related mechanical properties to the types of myosin that are present, and while Vmax is thought to be limited by the rate of dissociation of ADP from A.M.ADP (56), the molecular basis for differences in contractile behaviors of muscles with different MHC isoforms is not yet known. This knowledge is critical to ultimate understanding of the basis for adaptive alterations in muscle function and will contribute to improved understanding of contractile dysfunction in diseases such as congestive heart failure, in which the kinetics of contraction can be considerably slowed compared to normal parameters (13).